Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, small pore silicoaluminophosphates (SAPOs), such as SAPO-34 and SAPO-18, have shown particular promise. SAPO-34 belongs to the family of molecular sieves having the framework type of the zeolitic mineral chabazite (CHA), whereas SAPO-18 belongs to the family of molecular sieves having the AEI framework type. In addition to regular ordered silicoaluminophosphate molecular sieves, such as the AEI and CHA framework types, disordered structures, such as planar intergrowths containing both AEI and CHA framework type materials, are known and have shown activity as oxygenate conversion catalysts.
The synthesis of silicoaluminophosphate molecular sieves involves preparing a mixture comprising a source of water, a source of silicon, a source of aluminum, a source of phosphorus and at least one organic directing agent for directing the formation of said molecular sieve. The resultant mixture is then heated, normally with agitation, to a suitable crystallization temperature, typically between about 100° C. and about 300° C., and then held at this temperature for a sufficient time, typically between about 1 hour and 20 days, for crystallization of the desired molecular sieve to occur.
As is the case with the production of most synthetic molecular sieves, the synthesis of silicoaluminophosphates, and in particular intergrown forms thereof, must be closely controlled in order to avoid or minimize the production of impurity phases that can adversely affect the catalytic properties of the desired product.
According to the present invention, it has now been found that the order of addition of the starting materials, particularly of the aluminum source, and more particularly of both the aluminum source and the silicon source, and the temperature of the synthesis mixture during the addition of the starting materials can significantly impact the success of SAPO synthesis processes, particularly when conducted on a large, commercial scale. Thus, the organic directing agent used in the synthesis of silicoaluminophosphates is often a basic compound, such as a basic nitrogen compound, whereas an attractive phosphorus source is phosphoric acid or a similar phosphorus acid. The mixing of these materials can therefore generate heat and hence raise the temperature of the synthesis mixture. It has now been found that such a rise in temperature can lead to undesirable side reactions and possible production of impurity phases if the aluminum source is present in the mixture or is added thereto before the mixture has been allowed to cool.
U.S. Pat. No. 5,879,655 discloses a method for preparing a crystalline aluminophosphate or silicoaluminophosphate molecular sieve and teaches that it is critical, especially in large scale preparations, that at least some of the organic directing agent be added to the aqueous reaction mixture before a significant amount of the aluminum source is added, since otherwise the aluminum can precipitate to produce a viscous gel. In particular, the addition of the phosphorus source, aluminum source, and the organic directing agent to the aqueous reaction mixture is controlled so that the directing agent to the phosphorus molar ratio is greater than about 0.05 before the aluminum to phosphorus molar ratio reaches about 0.5.
According to Example 1 of U.S. Pat. No. 5,879,655, SAPO-11 can be produced from a synthesis mixture obtained by initially adding 17.82 kg of 86% H3PO4 to 8.59 kg of deionized ice in a stainless steel drum with external cold water cooling. 9.70 kg of aluminum isopropoxide and 21.0 kg of deionized ice are then added simultaneously in small increments over a time period of 1.5 hours with mixing using a standard mixing impeller and homogenization using a Polytron. 3.49 kg of di-n-propylamine are then added slowly with mixing. An additional 21.59 kg of aluminum isopropoxide and 18.0 kg ice are added in small increments over a time period of four hours with mixing/homogenization, followed by an additional 3.49 kg of di-n-propylamine. 2.30 kg of fumed silica (Cabosil M-5) are then added with mixing/homogenization until >95 weight percent of the particles in the mix are smaller than 64 microns. It is reported that, during the entire procedure, the temperature of the mixture never exceeds 30° C.
Example 2 of U.S. Pat. No. 5,663,480 discloses the synthesis of a crystalline titanoaluminophosphate from a mixture obtained by placing 34.6 g of phosphoric acid (85% by weight aqueous solution) into a beaker having a capacity of 500 ml and then adding 73.6 g of tetraethylammonium hydroxide (20% by weight aqueous solution). After stirring, the resulting mixture is cooled to room temperature, and to this mixture, 18.0 g of ion-exchanged water and 21.9 g of pseudo-boehmite are added, and then 15.8 g of titanium tetraisopropoxide is also added. After the contents of the beaker are stirred for 2 hours, the resulting mixed solution is poured into an autoclave to carry out hydrothermal synthesis.
International Patent Publication No. WO 02/70407, published Sep. 12, 2002 and incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown form of molecular sieves having AEI and CHA framework types, wherein said intergrown form has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the molecular sieve. Synthesis of the intergrown material is achieved by mixing reactive sources of silicon, phosphorus and a hydrated aluminum oxide in the presence of an organic directing agent, particularly a tetraethylammonium compound. The resultant mixture is stirred and heated to a crystallization temperature, preferably from 150° C. to 185° C., and then maintained at this temperature under stirring for between 2 and 150 hours. In the Examples, various sequences are disclosed for producing the EMM-2 synthesis mixture, in which the phosphorus source is initially combined with either the directing agent or the silicon source and then the remaining components, including the aluminum component, are added without prior cooling.
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is also said to be an AEI/CHA mixed phase composition. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown forms of AEI and CHA structure type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30. RUW-19 is synthesized by initially mixing an Al-source, particularly Al-isopropoxide, with water and a P-source, particularly phosphoric acid, and thereafter adding a Si-source, particularly colloidal silica and an organic template material, particularly tetraethylammonium hydroxide, to produce a precursor gel. The gel is then put into a steel autoclave and, after an aging period at room temperature, the autoclave is heated to a maximum temperature between 180° C. and 260° C., preferably at least 200° C., for at least 1 hour, with the autoclave being shaken, stirred or rotated during the entire process of aging and crystallization. Factors which are said to enhance the production of the mixed phase RUW-19 material include maintaining the SiO2 content of the gel below 5%, reducing the liquid content of the gel after addition of the SiO2 source and crystallization at temperatures of 250° C. to 260° C. Pure AEI and CHA phases are said to be favored at temperatures of 200° C. to 230° C.